Goal:
  
  A : Type
  v : A
  _a_ : A
  _l_ : list A
  n : nat
  H : hd_error (skipn n _l_) = Some v
  ============================
   (∃ n : nat, hd_error (skipn n (_a_ :: _l_)) = Some v)


DEBUG:Going to execute:
<coq-core.plugins.ltac::exists@1> $1

Goals:
    A : Type  v : A  _a_ : A  _l_ : list A
  ============================
   (hd_error (skipn 0 (v :: _l_)) = Some v)

  
  A : Type
  v : A
  _a_ : A
  _l_ : list A
  n : nat
  H : hd_error (skipn n _l_) = Some v
  ============================
   (hd_error (skipn (S n) (_a_ :: _l_)) = Some v)


DEBUG:Going to execute:
rewrite ?skipn_0 ?skipn_S => //=
DEBUG LTAC Evaluated term: (S n)